1. Field of the Invention
This invention relates to an ionization smoke detector device, and more particularly, to that class of devices which are arranged to have a radioactive source and a dual-region ionization chamber irradiated by such source.
As is well known, the principle of an ionization smoke detecting device relies upon the fact that when a gas contained within a limited space is ionized by radioactivity from a suitable source, such as radium or caesium, a resultant ion current can be measured. Changes in the composition of the gas due to the entry of smoke particles or the like can be detected by sensing or detecting a change in the ion current.
The present invention provides a solution to the difficulty which arises in the operation of certain ionization smoke detectors when fairly severe air movements within the detecting chamber take place due to strong gusts of wind or the like occurring in the ambient atmosphere. If not controlled, such air movements produce a false alarm because they simulate a smoke condition; that is to say, the air movements cause the same ionic flow change as smoke would cause, and hence have the same result of triggering an alarm.
Although the present invention will be described in the context of a particular form of ionization detector, it will be understood that the principle is applicable to other types and kinds of detectors.
2. Background Art
For an appreciation of a specific context to which the present invention is applicable, reference may be made to U.S. Pat. Nos. 3,935,492 and 3,935,466. The former describes an ionization smoke detector having two ionization chambers or regions, one of said chambers having an electrode surrounded by a conductive mesh, with a radioactive source being carried by said outer electrode. The latter patent, i.e. U.S. Pat. No. 3,935,466 discloses a similar arrangement, but utilizes a differently shaped intermediate electrode.
Reference may also be made, in connection with the concept of a dual region or dual chamber ionization detector, to U.S. Pat. No. 4,017,733. In that patent a similar scheme to that previously noted in U.S. Pat. No. 3,935,492 is proposed, according to which a cup-like outer electrode member is provided having circumferentially arranged openings or windows and with a plate or disc-like intermediate electrode subdividing the principal chamber into two regions or sections. The smoke sensor or detector of U.S. Pat. No. 4,017,733 has as one of its major objectives the prevention of spurious indications of smoke due, for example, to a comparatively intense air stream which might otherwise cause a change in the normally high impedance present in the chamber.
Another patent of interest, which happens to be referred to in U.S. Pat. No. 4,017,733, is U.S. Pat. No. 3,731,093. The smoke sensing detector therein described utilizes a cup-like, double-wall structure consisting of inner and outer sheathing walls formed with windows at respectively different levels to provide for an appropriate resistance to the flow of an incoming air stream. Accordingly, this U.S. Pat. No. 3,731,093 is also directed to reduce or to weaken the effects of an air stream within a sensing chamber upon ionic flow.
Whatever the merits of the devices or systems of the aforenoted patents, essentially no effective solution has been furnished by these patents with respect to providing a simple structure for a smoke detector, while achieving substantial reduction in undesired effects of air currents entering the ionization chamber.
Accordingly it is a primary object of the present invention to allow ready access of the ambient atmosphere into the smoke detecting chamber of a smoke detector device, while keeping the structure of the device extremely simple and low in cost.
Another basic object is to prevent a false alarm from being given in the situation where occasional strong winds would tend to cause a sharp change in the ionic flow, thus simulating the presence of smoke in the detecting chamber.